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Appendix VI.                          1137

opposite end. The important part of this mechanism is the
convex mirror E, which receives a ray of light from the acetylene
lamp c after reflection by prism D, and projects it upon B. The
larger drawings shew the mechanism more clearly. A gun-metal
casting F fixed to the box by the seat G, contains the plungers K
and L' whose motions represent engine stroke and pressure
respectively : the pressure movement being caused through the
buckling of a thin diaphragm M by the engine pressure in a small-
bore pipe N. The plunger K is actuated by the * reproducer' Q R s T,
so called because it imitates the engine stroke, and consisting of a
flexible shaft Q, connected at one end to shaft R, and at the other
to the crank shaft of the engine. Two equal spur wheels continue
the motion to shaft s, upon which is a crank pin and link to rock
the lever T. The proportions of these levers must be such as to copy
on the average the motion of the crank and connecting rods of
the engines usually indicated; and the bearings of the two shafts
are contained in the larger bush p, which may be rotated into any
required position round the centre s, by the milled wheel u and
worm v. The object of this change is the synchronisation of x's
phases with those of the engine stroke, which cannot be done till
the flexible shaft has taken its drive. The mirror is supported at
three points K, L and w, which together form a right-angled triangle ;
and the point w is a rigid fulcrum or pivot, but the supports K and
L are the springs j and H respectively. The plunger K, pressing
behind spring j, moves the mirror in a horizontal plane to represent
stroke ; while the plunger L, pressing behind spring H, moves the
mirror in a vertical plane, or at right angles to the former move-
ment, and indicates pressure. As the deflections of the plate M
are not exactly proportional to pressure, the back of the mirror is
so curved at the point / that the necessary compensation is
affected. The pipe N is connected to a three-way cock, of the
usual pattern, on the engine cylinder; and the shaft Q can be
connected while the engine is running.

When the speed is 1200 revs, per m. in a four-cycle motor,
the indicator diagram can be viewed as one picture on account of
the persistence of vision. It can then be traced on the plate.
But the best way is to insert a photographic plate in a dark slide,
and expose for one or two seconds, In this manner Prof.